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1.2. Halo Mass and Orbit Decay

Detailed simulations of systems like the Antennae may also provide useful constraints on dark halos. To produce proper tidal tails, disk material must escape to infinity. Very massive, quasi-isothermal halos prevent interacting galaxies from forming long tails (Dubinski et al. 1996); clearly, such halos are not present around galaxies like NGC 4038 / 9 or NGC 7252 (Mihos et al. 1998). But equally massive halos with density profiles falling off as rho2 propto r-3 at large r are not excluded, as N-body experiments explicitly demonstrate (Springel & White 1998, Barnes 1999). In sum, tail length tells us something about the structure of halos, but little about their total mass.

However, it seems unlikely that arbitrary amounts of dark mass can be included in simulations of interacting systems. The orbital evolution of a pair of galaxies is largely governed by the interaction of their dark halos (White 1978, Barnes 1988). Too much or too little orbital decay will hinder the construction of models which evolve from plausible initial conditions to configurations matching the observed morphologies and velocity fields of real systems. Possible indicators of halo mass in interacting systems include:

  1. Tail kinematics; the run of velocities along a tidal tail may constrain the potential.

  2. Tail fallback; if orbit decay is strong, returning tail material may miss the disk by a wide margin.

  3. Galaxy velocities; do the hulks preserve their original sense of motion about each other?

The last of these, in particular, seems relevant to NGC 4038 / 9; the galaxies must retain a good deal of their orbital angular momentum to produce the crossed tails emblematic of this system.